This invention relates to a process for producing carbon fiber, and more particularly it relates to a process for the industrial production of carbon fiber which process is typified by energy saving and improved operation stability in the production of carbon fiber.
Hitherto, carbon fibers have been produced by a process comprising an oxidation step in which precursor fibers such as acrylic fibers, cellulose fibers or pitch fibers are heated in an oxidizing gas atmosphere such as air of 200.degree. to 400.degree. C. and converted into oxidized fibers or infusibilized fibers (hereinafter referred to simply as oxidized fibers), and a carbonization step in which the oxidized fibers are further heated in a high-temperature atmosphere of inert gas such as nitrogen, argon, or helium and thereby converted into carbon or graphite fibers (hereinafter referred to simply as carbon fibers).
Gas exhausted from these oxidation and carbonization steps contains a large variety of compounds including oxidation and pyrolysis products of the precursor and oxidized fibers, and release of exhaust gas containing these compounds into the atmosphere presents serious problems concerning especially atmospheric air pollution and environmental hygiene. Further, high-boiling compounds such as pyrolyzates are contained abundantly in the high-temperature exhaust gases from the above oxidation and carbonization steps, and these compounds tend to condense to cause problems such as choke-up of piping unless the temperature of the gases is maintained higher than the condensation point of these compounds. Thus, industrial disposal of these gases has been a key point for the industrial production of carbon fibers.
The present inventors have pursued assiduous studies on the treatment of exhaust gas in the production of carbon fibers and, as a result, have found that the principal constituents of the exhaust gas in the oxidation step are oils released in the course of heat treatment of precursor and oxidation products, and these materials easily condense in an atmosphere below 200.degree. C. to form tar-like matter; that, since the weight loss on heating of the precursor itself in the oxidation step is usually about 3 to 5%, the calorific value of the gas exhausted from the oxidation step (hereinafter referred to as oxidation exhaust gas) is merely of the order of several tens of kcal/m.sup.3 under normal conditions (0.degree. C., 1 atm.) and hence such gas has little self-combustibility; and that, in the carbonization step in which the oxidized fibers formed in the oxidation step are heated in an inert atmosphere, there takes place a weight loss upon heating of at least about 40% by weight based on the weight of the precursor, and the gas exhausted from this carbonization step (hereinafter referred to as carbonization exhaust gas) is rich in pyrolyzates. Most of the pyrolyzates are produced in the recarbonization step in which the oxidized fibers are precarbonized and where the temperature is maintained in the range of 300.degree. to 900.degree. C., and the pyrolyzates produced in said precarbonization step contain materials (tar matter) which condense in an atmosphere of below 450.degree. C., while in the carbonization step where the temperature is maintained above 900.degree. C., only a small amount of pyrolyzates is produced and it contains essentially no such tar matter.
In the conventional methods for treatment of exhaust gases in the production of carbon fibers, the oxidation exhaust gas is usually made pollution-free by an oxidizing catalyst and then released into the atmosphere. However, when a silicone oil or an organosilicone is used as treating agent for the precursor fibers, since many of the pyrolyzates produced in this oxidation step have a poisoning action against the oxidizing catalyst, the catalyst life is shortened so that the catalyst must be frequently exchanged, resulting in a reduced working efficiency and an elevated cost for the catalytic treatment.
On the other hand, the gas exhausted from the carbonization step, that is, carbonization exhaust gas, is either similarly treated by an oxidizing catalyst and then released into the atmosphere, or burned by mixing with an oxidizing gas.
However, when a silicone oil or an organosilicone is used as treating agent for the precursor fibers, the catalytic treatment of the carbonization exhaust gas involves many problems because of the high content of pyrolyzates in the exhaust gas. That is, in addition to the problems found in the catalytic treatment of the oxidation exhaust gas as mentioned above, interruption of carbonization treatment is another problem which leads to an even greater deterioration of workability and greater loss of energy than are caused by interruption of the oxidation step. The burning treatment of the carbonization exhaust gas also presents a problem. Although the calorific value of the exhaust gas is in the order of several thousand kcal/m.sup.3 under normal conditions (0.degree. C./atm.) or higher, this exhaust gas has little combustibility because it contains no oxidizing gas. Therefore the exhaust gas can not be immediately subjected to the burning treatment. It is necessary to add an oxidizing gas such as air (which has been previously heated to a required temperature) to give the resultant gas mixture a sufficient combustibility for the burning treatment. In this case, in order to prevent condensation of the pyrolyzates contained in the exhaust gas, the oxidizing gas must be preheated before it is mixed with the exhaust gas. Naturally, additional equipment is necessitated for such a treatment. Thus, this type of method is very disadvantageous in its industrial applicability with respect to equipment cost and energy saving.
It is known that a carbon fiber manufacturing method using as precursor an acrylic fiber impregnated with silicone oil is capable of effecting smooth opening of fiber bundles when the precursor fibers are heated and oxidized in an oxidizing atmosphere to thereby convert them into oxidized fibers, and can provide carbon fibers having excellent properties with little tendency to cause fusion of individual fibers (Japanese Patent Laid-Open Nos. 10175/1978 and 131032/1979). However, when the precursor fibers impregnated with such silicone oil or so-called organosilicone are heated in an oxidizing atmosphere, vaporized silicone and pyrolyzates thereof are formed abundantly in the exhaust gas, and these products excessively shorten the life of catalyst, especially the oxidizing catalyst, to elevate the cost for the exhaust gas treatment. Further, once such products are accumulated in an exhaust gas treating system, it is extremely difficult to remove them by any cleaning means and, in some cases, the exhaust gas treating system itself becomes unoperative.